By way of example, audio units such as radios, cassette or record players or combinations of them, installed in vehicles, require from the user frequent adjustment of the so-called "volume" control so as to listen to the reproduced program with the same ease despite the ambient noise which, for its part, varies as a function of the various sources of noise, not only peculiar to the vehicle itself, namely engine, road/tire, aerodynamic and bodywork noise etc., but also as a function of the noises created by neighboring sources such as the other vehicles (for example trucks, etc.) or else due to the surrounding conditions (for example tunnels, etc.).
In order to alleviate some of these disadvantages, various devices or processes for automatic adjustment of volume have been proposed in the past based on automatic control of the gain of the amplifier and using either a microphone placed near the engine compartment or the vehicle's engine rev counter as noise measurement source. As is readily understood, such devices and in particular those described in U.S. Pat. No. 4,641,344, take into account only part of the so-called disturbing noises and, additionally, with engine compartments being ever better insulated, these devices are of only average effectiveness.
Moreover, whereas the rev counter gives a reliable indication of the engine noise, it gives no valid indication regarding the speed of the vehicle and hence the road and aerodynamic noises etc., which themselves become predominant on the road or motorway.
U.S. Pat. No. 4,641,344, also proposes taking information from each potential source of noise (speed, pressure, fan, rev counter, position of the windscreen wipers, of the steering wheel, condition of the road, etc.) but this entails many sensors and, therefore, much too high a cost. Additionally, this kind of device is peculiar to the vehicle in which it is installed.
Devices of the same kind as those described earlier have also been proposed, but based on measuring the subsonic part of the noise as, for example, the device described in U.S. Pat. No. 4,476,571. However, the user being sensitive to a wider range of frequencies, these devices are not very effective.
Furthermore, systems like those described, for example, in U.S. Pat. Nos. 3,497,622 and 4,628,526 have been proposed which measure, on the one hand, with the aid of a microphone, all the signals perceived by the user (reproduced signals+noise) and, on the other hand, the signals applied to the loudspeakers; a control signal is obtained representing the difference between these two signals (microphone and loudspeaker), which control signal is then used to modify the gain of the amplifier.
However, such systems are either sensitive to the blanks contained in the programs and hence have to be equipped with circuits for detecting these blanks, so as to avoid any adjustment due to the presence of said blanks (thus complicating the construction and raising the price; see U.S. Pat. No. 4,628,526) or require the presence of two amplifiers (see U.S. Pat. No. 3,497,622) which not only entails a higher cost but is also a potential source of problems since two amplifiers are in danger of operating in the opposite direction and hence of producing the reverse effect to that desired.
A further system disclosed in the publication "Konsumelektronik" is designed to cope with blanks by making use of the mean values of the microphone and radio signals. However, in this system, there is an adjustment factor which is determined by the maximum and minimum values of the microphone and radio signals and thus which is variable. ##EQU1## Use of an adjustment factor determined in this manner is a potential source of instability and thus means are provided to correlate successive values. As one can easily understand, such systems are rather complicated not only to work-out but also to properly adjust.
Moreover, a device such as that described by U.S. Pat. No. 4,628,526, which is based on the detection of the envelope of the signal, (derived signal) is extremely sensitive to the instantaneous variations in the noise and hence generates far too many corrections, quickly becoming unpleasant.
U.S. Pat. No. 4,677,389 proposes a device for distinguishing between real background noise and speech in a given environment (e.g., a car). However, the means proposed are rather complicated and require additional components and software that makes the device too expensive. Moreover, the means maintain the gain value fixed when speech is detected, although the real background noise might change and thus require an adjustment.